Various safety systems have been designed and implemented for automatically shutting in oil and gas wells. Such systems are used in the event of damage or malfunction of the wellhead, production facilities, pipelines, or the like, by fire, explosion or other accidents. Further, these systems may be automatically operated in the event of high or low flow line pressure changes. Generally, these safety valves are set below ground level in the casing or tubing string. However, such valves may also be used near or at the wellhead surface.
Use of such valves is of critical importance in reducing risk to human life and property which may result from uncontrolled oil or gas flow. Further, such valves are critical to reducing or eliminating emergency flow which could result in environmental damage. The use of such devices reduce the likelihood of needless lost of oil or gas which could occur in emergency situations.
A wide variety of designs have been employed in such safety valves. The most common designs are referred to as the flapper-type and ball-type. Poppet or gate valves have also been employed.
The ball-type safety valve design includes a spherical or near spherical valve having a port therethrough which when aligned with the flow channel of the tubing string permits flow therethrough. By rotating the spherical valve to interrupt the alignment, flow past the valve is arrested. Because of the necessary design of the ball-type valve, the port through the valve is substantially smaller in dimension than that through the production tubing. Thus, the valve introduces a restriction in the well tubing or casing which will not easily accommodate production tubing and tools or instruments necessary for production completion, testing or stimulation. Further, such valves do not lend themselves as readily to surface control.
Poppet valves or gate valves are also used in the design of safety valves, but these valves also introduce restrictions in the bore which do not easily permit the accommodation of tools and instruments, or production tubing for positioning or manipulation of such devices, below the safety valve.
Flapper-type valves have found substantial acceptance in the design of safety valves. In prior art flapper valves, a valve body supports a gate from a hinge which permits the valve to be pivoted against the wall of the valve and out of the way of the passageway through the valve. The valves are normally cammed open by the movement of a sleeve, under the action of hydraulic pressure, against the valve to pivot it to its open position. Closure of the valve is accomplished by the withdrawal of the actuating sleeve and the action of a return spring on the valve for pivoting the valve to its closed position. Further, closure of the valve is, in almost all cases, accompanied by a substantial pressure differential across the valve itself. Normally, flow is moving up the well bore, in the direction of the movement of the valve. Generally, this pressure differential may be on the order of thousands of pounds, and thus substantial forces can be exerted on the valve due to fluid velocity. As a result, substantial failure has been experienced in such valves, particulary at the hinge point due to high stress concentrations. This has been in part due to the uncontrolled closure and in view of the design which concentrates loading on the hinge pin area of the valve.
Thus, the need exists for a surface controllable safety valve which permits the controlled closure of the valve without the attendant loading and resultant stresses to which the valve and seat are subjected in prior devices. Further, this controlled closure must be accomplished by providing a design resulting in an unrestricted bore such that tools and instruments moved in the casing may pass through the valve as necessary.